1. Field of the Invention
The present invention relates to a torque converter for an automatic transmission having a lock-up clutch, and more specifically, to a slip control system for the lock-up clutch.
2. Description of the Prior Art
In the field of a torque converter incorporated in an automatic transmission of an automotive vehicle, a lock-up clutch mechanism has been proposed which provides a direct connection between an engine output member and an input member of the transmission under a predetermined vehicle operating condition which there occurs no amplification of a torque, or no change in torque from an engine so that fuel consumption performance of the vehicle is improved. When the lock-up clutch mechanism is engaged, the torque from the engine is transmitted so as to bypass the torque converter to the transmission through the lock-up clutch mechanism. Under this operation, it is advantageous in that the fuel consumption performance can be improved because there is no energy loss caused by a fluid friction in the torque converter. On the other hand, it is disadvantageous in that the torque amplification and a shock absorbing function of the torque converter cannot be obtained, thereby deteriorating a riding comfort.
In view of the above characteristics of the lock-up mechanism of the torque converter, Japanese Patent Public Disclosure (KOKAI) No. 62-297567, laid open to the public on Dec. 24, 1987, discloses a slip control system for a transmission of motor vehicle having a torque converter and lock-up clutch mechanism wherein the lock-up clutch mechanism is constituted by a converter cover connected with an engine output shaft and a damper piston connected with a turbine shaft, with the damper piston being brought into frictional engagement with the converter cover in accordance with a hydraulic pressure introduced into hydraulic chambers formed at front and back sides of the damper piston. The transmission is adapted to carry out a slip control in which the lock-up clutch allows a certain slippage so as to take selectively a lock-up condition, wherein an engine torque is transmitted through only the lock-up clutch mechanism, a converter condition, wherein the engine torque is transmitted through only the torque converter, and a slip condition, wherein the engine torque is transmitted through both the lock-up clutch and the torque converter.
Japanese Patent Public Disclosure No. 57-33253 (corresponding to U.S. Pat. No. 4,468,988 issued on Sep. 4, 1984) laid open to the public in 1982, discloses a slip control system for establishing a proper slip condition wherein respective rotation speeds of input and output members of the torque converter are detected and a hydraulic pressure for controlling the lock-up clutch is controlled so as to cause a rotation speed difference between the input and output members to converge to a predetermined value by means of a feedback control. U.S. Pat. Nos. 4,580,671, 4,618,041, 4,669,441, 4,687,083, and 4,468,988 also disclose conventional transmission control systems.
However, in the conventional slip control systems, it may take unduly long before the rotation speed difference between the input and output members converges with to a target value in the feed back control, due to a change in the input torque introduced into the torque converter. Thus, the conventional control is not responsive in obtaining a desirable slip condition.
If a feedback control gain for controlling the hydraulic pressure for the lock-up clutch is increased to accomplish a responsive slip control, a hunting of the control may occur, resulting in a deterioration of the stability of the slip control.
In a case where the hydraulic pressure is maintained at a constant value for obtaining the target speed difference, it is difficult to provide a proper slip condition corresponding to the input torque which changes in accordance with the engine operating condition and vehicle operating condition. In particular, in a transient condition or such as acceleration, deceleration of the vehicle and the like, the above problems would be remarkable.